lockfile can be used to create one or more semaphorefiles.
If lockfile can't create all the specified files (in the specified order), it
waits sleeptime (defaults to 8) seconds and retries the last file that
didn't succeed. You can specify the number of retries to do until
failure is returned. If the number of retries is -1 (default, i.e.,
-r-1) lockfile will retry forever.

If the number of retries expires before all files have been created,
lockfile returns failure and removes all the files it created up till that
point.

Using lockfile as the condition of a loop in a shell script can be done easily
by using the -! flag to invert the exit status. To prevent infinite
loops, failures for any reason other than the lockfile already existing are
not inverted to success but rather are still returned as failures.

All flags can be specified anywhere on the command line, they will be processed
when encountered. The command line is simply parsed from left to right.

All files created by lockfile will be read-only, and therefore will have to be
removed with rm-f.

If you specify a locktimeout then a lockfile will be removed by force
after locktimeout seconds have passed since the lockfile was last
modified/created (most likely by some other program that unexpectedly died a
long time ago, and hence could not clean up any leftover lockfiles). Lockfile
is clock skew immune. After a lockfile has been removed by force, a suspension
of suspend seconds (defaults to 16) is taken into account, in order to
prevent the inadvertent immediate removal of any newly created lockfile by
another program (compare SUSPEND in procmail(1)).

Suppose you want to make sure that access to the file "important" is
serialised, i.e., no more than one program or shell script should be allowed
to access it. For simplicity's sake, let's suppose that it is a shell script.
In this case you could solve it like this:

The behavior of the -! flag, while useful, is not necessarily intuitive
or consistent. When testing lockfile's return value, shell script writers
should consider carefully whether they want to use the -! flag, simply
reverse the test, or do a switch on the exact exitcode. In general, the
-! flag should only be used when lockfile is the conditional of a loop.

Calling up lockfile with the -h or -? options will cause it to display a
command-line help page. Calling it up with the -v option will cause it to
display its version information.

Multiple -! flags will toggle the return status.

Since flags can occur anywhere on the command line, any filename starting with a
'-' has to be preceded by './'.

The number of retries will not be reset when any following file is being
created (i.e., they are simply used up). It can, however, be reset by
specifying -rnewretries after every file on the command line.

Although files with any name can be used as lockfiles, it is common practice to
use the extension `.lock' to lock mailfolders (it is appended to the
mailfolder name). In case one does not want to have to worry about too long
filenames and does not have to conform to any other lockfilename convention,
then an excellent way to generate a lockfilename corresponding to some already
existing file is by taking the prefix `lock.' and appending the i-node number
of the file which is to be locked.